1. Field of the Invention
The present invention relates to stabilization zones for contact lenses requiring rotational stability, such as toric contact lenses, and more particularly to contact lenses requiring rotational stability and incorporating one or more surface modified zones in specific regions to generate a friction driven rotational force during blinking.
2. Discussion of the Related Art
Myopia or nearsightedness is an optical or refractive defect of the eye wherein rays of light from an image converge to a common focal point that is in front of the retina. Myopia generally occurs because the eyeball or globe is too long or the cornea is too steep. A minus powered spherical optic may be utilized to correct myopia. Hyperopia or farsightedness is an optical or refractive defect of the eye wherein rays of light from an image converge to a common focal point that is behind the retina. Hyperopia generally occurs because the eyeball or globe is too short or the cornea is too flat. A plus powered spherical optic may be utilized to correct hyperopia. In both myopia and hyperopia, the eye (cornea) is substantially spherical in shape, but the curvature is either too steep or too flat, thus causing the light rays of the image to converge in front of or behind the retina.
Astigmatism is a defect in the eye (corneal or lenticular) caused by a deviation from spherical curvature, which results in distorted images as the light rays from the image do not converge to a common focal point, but rather to a line of focus. Astigmatism can occur in combination with myopia or hyperopia and affects a large percentage of the population to some degree. A cylindrical optic, also known as a toric optic, rather than a spherical optic may be utilized to correct astigmatism. Accordingly, an individual with myopia or hyperopia as well as astigmatism would have a single optic with both spherical and cylindrical components.
A toric optic is an optical element having two different target powers in two orientations that are perpendicular to one another. Essentially, a toric lens has one power, known as the spherical power, for correcting the myopia or hyperopia component of refractive error, and a second power, known as the cylinder power, for correcting the astigmatism component, of which both powers are built into a single optical element. These powers are created with curvatures at different angles which are preferably maintained relative to the eye. Toric optics may be utilized in eyeglasses, intraocular lenses and contact lenses. The toric optics used in eyeglasses and intraocular lenses are held fixed relative to the eye thereby always providing optimal vision correction. However, toric contact lenses may tend to rotate on the eye thereby temporarily providing sub-optimal vision correction due to misalignment between the prescribed toric optics and the eye. Accordingly, toric contact lenses also include a mechanism to keep the contact lens relatively stable on the eye when the wearer blinks or looks around.
It is known that correction of certain optical defects may be accomplished by imparting non-rotationally symmetric corrective characteristics to one or more surfaces of a contact lens such as cylindrical, bifocal, multifocal, wavefront corrective characteristics or decentration of the optical zone. It is also known that certain cosmetic features such as print patterns, markings, and the like are required to be placed in a specific orientation relative to the wearer's eye. The use of contact lenses is problematic in that each contact lens of the pair must be maintained at a specific orientation while on the eye to be effective. When the contact lens is first placed on-eye, it must automatically position, or auto-position, itself and then maintain that position over time. However, once the contact lens is positioned, it tends to rotate on the eye due to the force exerted on the contact lens by the eyelids during blinking, eyelid and tear film movement and to some extent, gravity.
Maintenance of the on-eye orientation of a contact lens typically is accomplished by altering the mechanical characteristics of the contact lens. For example, prism ballast stabilization, including decentering of the contact lens' front surface relative to the back surface, thickening of the inferior contact lens periphery, forming depressions or elevations on the contact lens' surface, and truncating the contact lens edge, are all methods that have been utilized.
Additionally, static stabilization has been used in which the contact lens is stabilized by the use of thick and thin zones, or areas in which the thickness of the contact lens' periphery is increased or reduced, as the case may be. Typically, the thick and thin zones are located in the contact lens' periphery with symmetry about the vertical and/or horizontal axes. For example, each of two thick zones may be positioned on either side of the optic zone and centered along the 0-180 degree axis of the contact lens. In another example, a single thick zone positioned at the bottom of the contact lens providing a similar weight effect, like that of prism stabilization, but also incorporating a region of increasing thickness from top to bottom in order to utilize upper eyelid forces to stabilize the contact lens may be designed.
The challenge with static stabilization zones is a tradeoff between contact lens stability and comfort, plus the physical limitations associated with increased thickness. With a static stabilization zone, the slope of the stabilization zone is fixed in the contact lens. Changes to the design to improve rotational speed, such as increasing the surface slope of the stabilization zone, also increases contact lens thickness and may adversely impact comfort. Additionally, the contact lens design has to accomplish two things; namely, to rotate to the proper orientation on insertion, and to maintain that orientation through the wear period. A static design requires tradeoffs in performance between these two modes.
As individuals age, their eyes are less able to accommodate, or bend their natural or crystalline lens, to focus on objects that are relatively near to the observer. This condition is known as presbyopia. More specifically, when an individual is born, the crystalline lens is pliable which makes it capable of a high degree of accommodation. As the individual ages, the crystalline lens gradually becomes more rigid and thus less able to accommodate. Similarly, for persons who have had their natural or crystalline lens removed and an intraocular lens or IOL inserted as a replacement, the ability to accommodate is absent. Although the intent of an accommodating IOL is to address this potential shortcoming, current accommodating IOL designs and concepts are relatively new and continue to evolve.
Among the methods used to correct for the eye's failure to accommodate is a method known as mono-vision in which, in most cases, a contact lens for correction of distance vision is utilized in the lens wearer's dominant eye, which is known to predominate for distance vision, coupled with a second contact lens for correction of near vision being utilized in the non-dominant eye. Mono-vision provides for both near and distance vision while allowing the brain to compensate as how the images should be construed. Another known method for correction of presbyopia is to use bifocal or multifocal contact lenses in both of the individual's eyes. There are many forms of bi-focal or multi-focal contact lenses for the correction of presbyopia. These design forms include concentric rings and aspheric designs, both of which may be designed for center distance or center near. All of these designs function by providing a range of powers within the pupil of the eye. For example, a concentric ring design may have a central ring that provides powers that are nominally equal to the power required to correct the distance vision of the subject, an adjacent ring that provides near powers, and an outer ring that also provides distance powers. There may also be fitting strategies to address intermediate vision demands between near and far distances, for example, computer screen viewing. Use of bifocal or multifocal lenses in both eyes results in a reduction of image contrast and resolution compared to mono-vision but usually maintains binocularity. Yet another method of treating presbyopia is to place a bifocal or multifocal lens in one eye and a single vision lens in the other eye. The disadvantage in using this method is in the large number of lenses that must be considered in order to provide the individual with satisfactory lens performance and the limitation of binocularity at near distance.
With respect to contact lenses for treating presbyopia, bifocal or multifocal lenses require translation of movement on the eye. Whereas the astigmatic lenses described above require centration and rotational stabilization, lenses for treating presbyopia require a more linear movement in order to keep the required power in the correct place for near, intermediate and distance vision. The means for this translation has taken a number of forms, including lens truncation and thicker zones located in specific regions to cause translation of the lens as the gaze of the wearer shifts. Once again comfort has to be balanced against movement.
Accordingly, it would be advantageous to design a contact lens with surface modified zones that maintain orientation and stability performance while providing the comfort of a single vision lens. The surface modified zones are preferably designed with a balance between the ability to cause rotation versus comfort.